Conclusions.:
The diurnal variation of the TD-OCT measurements was likely due to the limited repeatability of the device rather than to tissue variation. Diurnal variation was not found using SD-OCT, which has better repeatability.

Measuring macular thickness is critical for evaluating the severity of macular disease and follow-up treatment success. Macular thickness can be examined with retinal thickness analyzers, confocal scanning laser ophthalmoscopes, and optical coherence tomography (OCT). OCT is a widely used, noninvasive technique that can show each retinal layer using backscatter light with an 850-nm diode laser and can be used to observe the structure of the retina directly.1–3

Time domain (TD) OCT collects information about retinal thickness using the time interval of backscatter light movement in a reference mirror. The recently developed spectral domain (SD) OCT uses the time interval of backscatter light reflected in a reference mirror through an interferometer; unlike TD-OCT, however, the reference mirror is fixed. When passing through the interferometer, the backscatter light forms a spectrum that is received by the spectrometer. The images used for later measurements are made through Fourier transformation. SD-OCT has a better axial resolution than TD-OCT, resulting in more precise internal retina structure imagery that provides detailed information. Moreover, it has better repeatability than TD-OCT.4,5

Several studies report diurnal variations in macular thickness in macular disease patients.6–8 Thus, we used TD-OCT and SD-OCT to investigate the diurnal variations in macular thickness in healthy subjects without thickened retinas caused by macular disease and compared the variations measured by the devices.

Subjects, Materials, and Methods

This was a prospective cohort study. The protocol was approved by the institutional review board of Chungnam National University Hospital. All participants signed informed consent forms, and the study adhered to the tenets of the Declaration of Helsinki.

Subjects

Between September 2009 and January 2010, 52 eyes in 52 subjects with no systemic or ocular disease and no history of ocular disease, including refractive surgery, were examined. By random selection, 29 right (55.7%) and 23 left (44.3%) eyes were examined. The subjects consisted of 25 men and 27 women. Uncorrected visual acuity and best-corrected visual acuity (BCVA) were tested, and slit lamp examinations and fundus examinations were conducted to investigate the condition of each retina. OCT was performed at 8 AM and 6 PM. In all subjects, the intraocular pressure (IOP) was <21 mm Hg, the spherical equivalent refraction was ±4D, astigmatism was ±2D, and BCVA was greater than 1.0. Slit lamp examinations and fundus photographs showed no specific findings.

TD-OCT

A Stratus OCT (Carl Zeiss Meditec, Dublin, CA) was used for TD-OCT. Macular thickness and macular volume were measured using the fast macular thickness map protocol, consisting of six radial scans, including 1024 axial pixels and 512 transverse pixels, centered at the fovea centralis, with a 30° gap between scans. The images were analyzed with measurements of the central 20° of the visual field on a 6-mm diameter map. The macular thickness map contained three concentric rings of 1-, 3-, and 6-mm diameters. Each outer ring was divided into four parts for a total of nine regions. Average macular thickness and total macular volume were examined (Fig. 1).9 Only scans with signal strengths greater than 7 were included.10

SD-OCT was performed with a Cirrus HD-OCT (Carl Zeiss Meditec). Macular thickness and macular volume were measured with a macular cube 128 × 512 combination scanning pattern, which used the raster scan method to scan a 6-mm2 macular area with 128 × 512 points (width × length). Images were analyzed with measurements of the central 20° of the visual field on a 6-mm diameter map. The macular thickness map contained three concentric zones. Each outer zone was each divided into four parts for a total of eight regions. Then the average macular thickness and total macular volume were examined.9 Only scans with signal strengths greater than 7 were included.10

Measurements

One experienced examiner performed TD and SD-OCT consecutively at 8 AM and 6 PM of the same day to measure macular thickness and macular volume.

Because the terminology varies in OCT studies, we define here the terms used: fovea, central 1-mm circle area in a circular macular thickness map; central macular thickness, thickness of the fovea; pericentral macular area, inner (between 1-mm central ring and 3-mm ring) and outer (between 3-mm and 6-mm rings) rings in a circular macular thickness map, consisting of the temporal, nasal, superior, and inferior inner and temporal, nasal, superior, and inferior outer circle areas; pericentral macular thickness, thickness of the inner and outer rings; absolute variation, the difference in the thickness of a parameter at two different times. For example, if M1 was measured at 8:00 AM and M2 at 6 PM, then the absolute variation equals (M1 − M2). The absolute change in thickness is the first of three methods of analyzing the OCT changes shown here; relative variation, absolute variation divided by the initial thickness measurement. Using the same symbols, relative thickness equals [(M1 − M2)/M1]; modulus of absolute variation, the difference in thickness between two measurements made at different times, disregarding its sign. Continuing the example above, the modulus of absolute variation equals |M1 − M2|; modulus of relative variation, the absolute change in thickness divided by the baseline thickness, disregarding its sign. The modulus of relative variation equals [|M1 − M2|/M1].

Statistical Analysis

Statistical analyses were conducted using SPSS software (version 12.0; SPSS, Chicago, IL). Morning and afternoon measurements were analyzed using a paired t-test. Absolute and relative variations of each device in the morning and afternoon were calculated and compared. Statistical significance was defined as P < 0.05.

Results

Of the 52 subjects, 25 (48.1%) were men and 27 (51.9%) were women ranging in age from 25 to 75 years (mean, 49 ± 19 years). After random selection, 29 (55.7%) right and 23 (44.3%) left eyes were examined (Table 1). The central macular thickness measured with TD-OCT was significantly higher at 6 PM (193.8 ± 18.3 μm) than at 8 AM (190.3 ± 16.1 μm; P = 0.006). The increase in total macular volume between 8 AM and 6 PM was not statistically significant. For the pericentral macular thickness measured using TD-OCT, the superior and temporal inner area thicknesses increased significantly from 273.0 ± 14.4 to 275.5 ± 16.2 μm and from 257.5 ± 16.7 to 260.8 ± 18.0 μm, respectively (P = 0.010, P = 0.006; Table 2).

Conversely, the central macular thickness measured using SD-OCT did not differ significantly between the morning (241.5 ± 20.0 μm) and the afternoon (241.8 ± 19.4 μm). Furthermore, there was no significant difference between the morning and afternoon total macular volumes or pericentral macular thicknesses (Table 3).

Comparing the absolute variation of the two devices, central macular thickness differed significantly, from −3.437 for TD-OCT and −0.250 for SD-OCT (P = 0.015); the negative value indicated a higher afternoon measurement. The absolute variation of the temporal inner area thickness was −3.375 for TD-OCT and −0.375 for SD-OCT, also statistically significant. The absolute variations of the other pericentral macular areas and the total macular volume did not differ significantly between the two devices (Table 4).

The relative variations of the central macular thickness measured with TD-OCT (−0.0180) and with SC OCT (−0.0014) were significantly different (P = 0.014). Although the relative variations in the temporal inner area thickness differed significantly between TD-OCT (−0.0132) and SD-OCT (−0.0016; P = 0.027), none of the other area thicknesses or the total macular volume relative variations were significantly different (Table 5).

The modulus of absolute variation was significantly lower with SD-OCT than with TD-OCT for all parameters, except total macular volume (Table 6). The modulus of relative variation was also significantly lower with SD-OCT than TD-OCT for all parameters (Table 7).

Until recently, TD-OCT was used widely to gauge macular thickness. It has a speed of 400 A-scans per second and an axial resolution of 10 μm. The newly developed SD-OCT is more than 50× faster, with more than 20,000 A-scans per second with an axial resolution of 5 μm. Although the working principles of the two devices are similar, TD-OCT measures signals by time by moving the reference mirror, and SD-OCT detects the light spectrum from the interferometer with a spectrometer and a fixed reference mirror to obtain retinal thickness information more rapidly. Additionally, because TD-OCT collects data along six radial lines crossing the macular center, only 5% of the total macular area is examined, whereas macular thickness is estimated by extrapolation.

SD-OCT uses raster scanning by forming horizontal scan lines from the top down to form the image, which can collect more data. This method shows the inner retina structure more precisely than TD-OCT.11,12

The built-in auto-algorithm in TD-OCT uses the distance from the internal limiting membrane to the inner segment and outer segment junction of the photoreceptor layer as the retinal thickness. Conversely, SD-OCT can differentiate the retinal pigment epithelium (RPE) from the inner segment and outer segment junction of the photoreceptor layer because of its higher resolution; thus, the RPE is designated as the posterior retinal boundary. This is why retinal thickness measured using SD-OCT is 40 to 50 μm thicker than when using TD-OCT and is a better approximation of actual retinal thickness.9,13

Many studies have compared TD-OCT and SD-OCT macular thickness measurements. Leung et al.13 investigated the repeatability of both devices on healthy subjects. They calculated the coefficient of variation (CV) by dividing the SD by the mean and found that the TD-OCT and SD-OCT CVs were 1.62–3.21 and 0.58–2.42%, respectively. Kakinoki et al.9 determined that the TD-OCT and SD-OCT CVs were 0.7% to 3.3% and 0.2% to 1.3%, respectively. Forooghian et al.14 found that the CVs in DME patients were 0.86% to 3.13% and 0.58% to 2.87% for TD-OCT and SD-OCT, respectively. These studies suggest that although the CVs of both devices are good, SD-OCT is better than TD-OCT.9,13,14

There are several current reports on the diurnal variation of retinal thickness among patients with macular edema. According to Frank et al.,7 central macular thickness decreased gradually over time in 10 DME patients; a higher initial value was associated with a greater decrease. Another study measured diurnal variation in 156 eyes of 96 DME patients using TD-OCT. The average central macular thickness in the morning was 368 μm, which decreased by 6% to 13 μm in the evening.6 Gupta et al.8 revealed that central macular thickness decreased from 571 to 475 μm over time in 10 central retinal vein occlusion patients. These changes were assumed to follow changes in blood pressure, retinal metabolism, and standing position. None of these studies investigated diurnal variation in healthy subjects.

This study observed diurnal variation of macular thickness in 52 eyes of 52 healthy persons conducting daily activities healthily, with no underlying disease. The central macular thickness measured with TD-OCT was 3.5 μm thicker in the afternoon than in the morning (P = 0.006). Additionally, the macular thickness of the superior inner area and temporal inner area was 2.5 μm (P = 0.01) and 3.2 μm (P = 0.006) greater, respectively, in the afternoon than in the morning. Other areas showed insignificant increases. The central macular thicknesses measured using SD-OCT did not differ significantly in the morning and afternoon (0.3 μm; P = 0.71). Furthermore, there was no significant difference in macular volume or pericentral macular thickness between the morning and the afternoon.

Comparing the absolute variations of the devices revealed that the central macular thickness variation was significantly lower using SD-OCT than TD-OCT (P = 0.015). The temporal inner area thickness was also significantly lower with SD-OCT than TD-OCT (P = 0.029). The other area thicknesses and macular volume showed no significant difference. Furthermore, the relative variations of the SD-OCT and TD-OCT measurements differed significantly for central macular thickness and temporal inner area, but not for the other areas or macular volume.

Mean variation of thickness is calculated by the summation of plus and minus values. To make the changes of measurement by machine more precisely, we used modulus of absolute variation. The modulus of variation was the difference in thickness between two measurements made at different times, disregarding its sign. Comparing the modulus of absolute variation showed that the modulus of SD-OCT was significantly lower than that of TD-OCT for all measurements except macular volume. The modulus of relative variation was significantly lower in SD-OCT than in TD-OCT for all measurements.

This study analyzed the diurnal variation of central macular thickness in healthy persons using TD-OCT and SD-OCT. Limitations of the study include the small number of subjects, all of whom were Koreans. Studies using more subjects and various races are necessary.

In conclusion, central macular thickness measured with TD-OCT was observed to be thicker in the afternoon than in the morning; however, SD-OCT did not show this diurnal variation, suggesting that diurnal variation in the TD-OCT measurements was caused by a repeatability limitation of the device rather than by actual variation. This error was reduced using SD-OCT, which has better repeatability.